Naegleria fowleri is a free-living thermotolerant amoeba in the phylum Sarcomastigophora that is distributed worldwide, mainly in warm aquatic environments. It has also been isolated in soil and sewage. It cannot tolerate nor has been isolated from seawater.
It exists in trophozoite and cyst forms and in a transient flagellate stage. N fowleri is the etiologic agent of primary amebic meningoencephalitis (PAM), which is a rare and usually fatal disease.
Carter and Fowler, for whom N fowleri is named, reported the first cases of PAM in 1965 in Australia, although these 4 cases were initially mistakenly attributed to Acanthamoeba infections.
Most patients with PAM have a history of water exposure that includes swimming or diving through a body of fresh water. In arid climates, cases have arisen from the inhalation of cysts. Trophozoites or cysts, which give rise to trophozoites after they excyst, penetrate the nasal mucosa and ascend along the olfactory nerves after phagocytosis by sustentacular cells of the neuroepithelium and pass through the cribriform plate to invade brain tissue with resultant purulent meningitis and encephalitis.
The organism produces its pathologic effects by its aggressive phagocytic activity and by the diffusion of phospholipases, neuraminidases, and other cytologic enzymes, which results in diffuse hemorrhage and necrosis of brain tissue.
Focal myocarditis with neutrophilic infiltrates and no visible amoebae has been reported, but its significance is still questionable.
In the US: Although isolation of N fowleri from the nares of asymptomatic individuals is reported, PAM itself is rare; only 31 cases were reported to the Centers for Disease Control and Prevention (CDC) from 1989-2002.
The risk of infection has been estimated at one case per 2.6 million exposures. Internationally: Approximately 200 cases of PAM have been reported worldwide, with some well-publicized outbreaks related to a single source. Most cases occur during the warm summer months in individuals who swam in freshwater pools or lakes.
